After second year male: The head and auriculars are olive-coloured and contrast with the strikingly yellow face. The throat and upper breast are jet black. Black streaks pass down the flanks to the vent. The lower breast and vent are usually white, washed with pale yellow. The upper and lower back is olive-coloured, not contrasting with the head. The folded wings appear grey or nearly black except for two white wingbars formed by the white tips of the greater and secondary coverts. The flight feathers are grey to dark grey with the primary coverts edged green. The folded tail appears grey or blackish. The outer tail feathers are grey with large white patches on all but the innermost (Figure 1).

After second year female: Resemble males but are easily separated in the field by over-all duller plumage and nearly white to mottled black throat. The black on the breast and flank streaking is reduced compared to the male. The white spots of rectricies and smaller and fewer.

Second year male: Appear mostly like adult males when at QUBS. They typically have reduced black on the throat, worn rectrices and flight feathers, brownish primary coverts that lack greenish edging and have much reduced white in the rectrices.

Second year female: The palest adult plumage of this species. There is very little black on the throat with hardly any grey streaking on the flanks. White on the rectrices is confined to small patches on the outermost feathers. Other wing and tail characteristics are as for second year male.

Juvenile (both sexes): Very similar to second year female but browner overall, especially on the under parts. This plumage is seen for a brief period after fledgling while still being provisioned by the parents (Pyle 1997; Morse and Poole 2005). The timing and location of the first basic moult is not certain but a small number of field observations suggests that at least some birds undergo this moult in July at QUBS (Conboy, personal observation).

Similar species: No other similar species regularly occur at QUBS. However vagrant species from western North America are occasionally observed in Ontario. Hermit Warbler has been recorded from Kingston (Weir 2009) and Townsend’s Warbler and Black-throated Grey Warbler have been recorded in Southern Ontario (Ontario Birds Listserv Archives). Vagrants should always be considered when identifying Black-throated Green Warblers. See Pyle (1997) and Sibley (2000) for identification and separation of these species.

Egg: 3-5 (usually 4) per clutch. 16.0 × 12.7 mm and roughly oval shaped. Shell is smooth and often glossy. The base colour is white, cream or greyish and marked with brown or reddish-brown spots and scrawls. Maculation (spotting) is usually confined to the wide end of the egg but there is a great deal of variation in the degree and position of maculation. Some eggs may be heavily marked nearly all over while others have light marks that are restricted to the very tip of the wide end. Maculation can take the form of small dots or large blotches and scrawls (Harrison 1975; Harrison 1978). Eggs may be confused with some other small passerines; however the wreathing of maculation at the larger end and a general lack of markings near the tip is somewhat unique amongst passerines with similar nesting habits and nest constructions at QUBS. One important exception is American Restart which may have near identical nest placement.

Nestling: Altrical and slightly downy at hatching. The skin is dark orange, the down is dark brown, the interior of the mouth is pink and the gape is light yellow (Baicich and Harrison 1997). Nestlings cannot be reliably distinguished from some other small passerines until they begin to grow juvenile plumage.

Vocalizations: Black-throated Green Warblers have two song types (Morse 1970), which only the males typically sing. The unaccented ending is usually described as zee-zee-zoo-zoo-zee. It is given with greater frequency later in breeding season once males have mated. The accented ending song is usually described as zee-zee-zee-zee-zoo-zee. It is given early in the breeding season and gradually declines in frequency as the unaccented ending song takes over. At QUBS songs are easily recognized, but call notes of Black-throated Green Warblers are difficult to distinguish from some other wood warblers. The most common call is usually described as a high-pitched see, although this call is lower pitched and thicker than the common calls of Pine Warbler, and sharper than the common call notes of Yellow-rumped Warbler. Calls may be given by males between songs or by either sex while in flight and during aggressive encounters with conspecifics and predators. Males often include a rapid series of call notes in the introduction of their unaccented songs.

Figure 2. Map of the breeding and wintering ranges of black-throated green warbler. From Birds of North America (Morse and Poole 2005).

Range: Endemic to the Americas (Figure 2), however vagrants have been found in Britain, Greenland, Iceland and Germany in autumn (Lewington et al 1991).

Breeding: Chiefly a bird of eastern North America but also extends into the southern boreal forest as far west as eastern British Columbia. Dendroica v. virens Breeds in the southern boreal and temperate zones of Newfoundland, the Maritimes, Quebec, Ontario, Manitoba, Saskatchewan, Alberta and British Columbia; also throughout the eastern United States from the Canadian border as far south as Alabama and the Arkansas Ozarks. D. v. waynei is restricted to the Atlantic coastal plain from Virginia to South Carolina (Morse and Poole 2005).

Wintering: D. v. virens typically winters from Mexico and throughout Central America. Some winter in the West Indies and northern South America. D. v. waynei probably winters in Cuba. There are winter records for southern Texas and Florida (Morse and Poole 2005). There are no winter records for the Kingston region (Weir 2009).

Habitat:

Breeding: Most of the literature describes Black-throated Green Warblers in association with a variety of conifer-dominated or mixed conifer-hardwood habitats (i.e. Stewart and Aldrich 1952; Morse 1978; Collins 1983). However, in some parts of its range Black-throated Green Warbler is found in pure deciduous forests (Brooks 1940; Kendeigh 1946; Rodewald and Smith 1998). It has been traditionally associated with eastern hemlock (Tsuga canadensis) in the Kingston region (Weir 2009). At QUBS Black-throated Green Warblers are found in both pure hardwood and mixed conifer-hardwood forests. Fine-scale systematic vegetation sampling of 17 Black-throated Green Warbler territories showed that 8 had no conifer component while an additional 5 had no eastern hemlock. Eastern hemlock was the dominate tree species (by basal area) in only 1 territory (Conboy unpublished data). Field observations suggest that sites which have a large hemlock component are occupied by the first migrants to return in spring. Hemlock-rich sites are not always filled up by the time warblers begin to set up territories in hardwood sites, suggesting that habitat limitation does not drive birds into “unsuitable” deciduous habitat (Conboy personal observation).

Wintering: In most of Central America black-throated green warblers are found at elevations of 300-2500 m, although sometimes nearer sea level as in the Yucatan. They inhabit mixed conifer-hardwood or pure hardwood and sometimes conifer plantations (Morse and Poole 2005).

Ecology:

Migration phenology: Black-throated Green Warblers are usually the third wood-warbler to arrive at QUBS in spring (after Pine [Dendroica pinus] and Yellow-rumped Warblers [D. coronata]). Males arrive on the breeding grounds a few days before females (Morse and Poole 2005). The mean arrival date for the Kingston region over the past 55 years is April 30 and the absolute earliest is April 17 (in 1981 and 2002 both at Prince Edward Point Bird Observatory; Weir 2009). The spring arrival date in the Kingston region has advanced by a mean of 3 days since 1988 based on data from Weir (1989, 2009). There are no long term records for arrival at QUBS but in 2008 the first Black-throated Green Warbler detected at the station was found on April 21 and in 2009 on April 23 (Conboy personal observation). Autumn migration begins in late August as birds begin to arrive from farther afield and local breeders abandon their territories. The mean last date in autumn over the past 51 years is Oct 4 and the absolute latest is November 8 (in 1992 at Treasure Island; Weir 2009). The mean last date for black-throated green in the Kingston region has advanced by 1 day since 1988 based on data from Weir (1989, 2009).

Food: On their breeding grounds Black-throated Green Warblers are primarily predators of caterpillars. Other invertebrates that form a smaller part of the diet include adult moths, beetles, homopterans, dipterans, spiders, mites and plant lice (Morse and Poole 2005). No specific work has been done on the diet of Black-throated Green Warblers at QUBS but field observations indicate that setaeless caterpillars form at least part of their diet (Conboy personal observation). On the wintering grounds and during migration Black-throated Green Warblers take insects and some fruit (Morse and Poole 2005).

Figure 3. Schematic of a coniferous tree that illustrates the feeding position of black-throated green warblers. The left side of the tree is the percent of total seconds of observations warblers spent in each section (n=2611). The right side of the tree is the percent of total observations of warblers in each section (n=164). The zones with the highest concentration of feeding activity are shaded. From MacArthur (1958).

Foraging: Most foraging probably takes place during the morning and late afternoon, but Black-throated Green Warblers will forage throughout the day during the breeding season, especially when provisioning young. Compared to other Dendroica Black-throated Green Warblers are among the most active foragers. In conifers they feed primarily in densest parts of branches, including around new buds. Foraging is centred midway up the tree (Figure 3). Black-throated Green Warblers glean insects from the branches they are perched on and those directly above them (MacArthur 1958). There is no similar information for foraging patterns in deciduous trees. They sometimes hover at the tips of branches or flycatch (Morse and Poole 2005). During the breeding season adults tend to forage alone, except when fledglings are present (Morse 1970) and when the male attends the female while she during off-bouts of incubation. During migration Black-throated Green Warblers are often found in mixed species flocks with other wood-warblers and insectivorous birds.

Territories: Breeding territories are established from late April to late May at QUBS. Most males are probably on territory by May 29, at which time 90% of black-throated green warblers have migrated through Prince Edward Point Bird Observatory on Lake Ontario (unpublished data). Males establish territory boundaries by singing and engaging in chases with other males. Males defend territories throughout the breeding season by singing, primarily in the early morning. Territories are used for foraging, nesting, roosting and all other activities. Both males and females spend nearly all of their time on their territory until it is abandoned when the young become independent. Some males at QUBS sing into mid-September, but it is unclear whether or not they are maintaining a territory so late in the season (Conboy personal observation). In the literature territory size ranges from 0.25-0.9 ha (Kendeigh 1945; Morse 1976b). Territories at QUBS range in size from 0.33-5.43 ha with a mean of 2.03 ha (n=21; Conboy unpublished data). Perhaps the range in territory sizes is the result of differing habitat quality, but that remains untested.

Territorial interactions: Black-throated Green Warblers usually dominate in territorial interactions with other wood-warblers (Morse 1967, 1976a). They frequently react to Cerulean Warbler (Dendroica cerulea) song playback and have been observed attacking and chasing live and attacking decoy Cerulean Warblers at QUBS (Conboy personal observation). Even though Black-throated Green Warblers are aggressive toward other wood-warblers there is little evidence of real interspecific territoriality (Morse 1968). Black-throated Green Warblers overlap their territories with other wood-warblers at QUBS (Figure 4): 26.32% of territories overlapped with Cerulean

Interspecific interactions: In 1994 a male Prairie Warbler (D. discolor) was discovered near Lake Opinicon singing the repertoire of both Prairie and Black-throated Green Warblers. This bird sang accented and unaccented songs of Black-throated Green Warbler that were aurally and sonographically identical to Black-throated Green Warblers recorded elsewhere. The prairie warbler also sang normal versions of its conspecific song (Martin et al 1995). There was no evidence of hybridization. Martin et al (1995) suggested that male Black-throated Green and Prairie Warblers may both have acted as social tutors for this bird when it was a nestling.

Predators and parasites: We have no little information on the predators and parasites of Black-throated Green Warblers at QUBS specifically. Known predators of D. virens include small mammals and birds including Blue Jays (Cyanocitta cristata) and Accipiter sp. (Morse and Poole 2005). Brown-headed Cowbird (Molothrus ater) is the only known brood parasite (Morse and Poole 2005); there are no records of cowbird parasitism in Black-throated Green Warblers nests at QUBS. There appears to be no information on other types of parasites.

Hybridization: Hybridization recorded with Townsend’s warbler in British Columbia (Rohwer 1994) and Black-and-White Warbler in the Appalachian Mountains (McCarthy 2006).

Breeding: Socially monogamous. Evidence of extrapair copulations is lacking but is thought to occur (Morse 1970). Courtship displays are poorly known (Ficken and Ficken 1962). Male plumage characteristics are assumed to be an important sexual signal for Black-throated Green Warblers but no research has been completed to confirm this. Typically only 1 clutch of eggs is laid per season. However if the first clutch is destroyed a second nesting attempt may be made. The nest is constructed by the female. Most nests found at QUBS have been placed in ironwood (Ostrya virginiana) and two have been found in white birch (Betula papyrifera); Martin personal communication). Will scold predators near the nest or fledglings or give a distraction display resembling a young bird that is unable to fly (Morse and Poole 2005).

Moults: Black-throated Green Warblers moult twice during the year producing a basic and an alternate plumage. The basic plumage (= non-breeding) and alternate plumage (= breeding plumage) are similar, but alternate plumages for both sexes tend to be brighter. The pre-basic moult is complete but the pre-alternate moult is partial with only some of the throat, upper body and flight feathers are renewed (Pyle 1997). Among our local breeders the pre-basic moult takes place at QUBS while the pre-alternate moult occurs on the wintering grounds. Most adult birds can be sexed in the field without capture. Aging is also often possible through binoculars, but is assured with the bird in hand, either through plumage characteristics or skull ossification patterns (in autumn only). Males are slightly larger than females, however plumage characteristics are more reliable for sexing.

Conservation status: QUBS status: Black-throated Green Warbler is a common species at QUBS (Conboy and Martin 2009) and throughout Eastern Ontario where sufficiently large tracts of forest exist. Based on 188 point counts across all QUBS tracts in 2008 and 2009, Black-throated Green Warbler was the 6th most common wood-warbler (after Ovenbird [Seiurus aurocapilla], Black-and-White Warbler, Yellow Warbler, Yellow-rumped Warbler and Common Yellowthroat [Geothlypis trichas]) and the third most common Dendroica (Conboy unpublished data).

Regional status: Considered “fairly common” by Weir (2009). Estimates of the number of Black-throated Green Warblers in the Kingston region have increased between the 1981-85 and 2001-05 Ontario Breeding Bird Atlases from 200 to 500±125 pairs (Weir 2009).

Research needs: For such a common and widespread species there are still many gaps in our knowledge of the Black-throated Green Warbler’s basic natural history. There is only limited information on breeding biology including displays, success rates, nest composition and extra-pair copulations. We also have very little information about migration and wintering ecology of Black-throated Green Warblers. The distinct use of either a conifer or a hardwood habitat by Black-throated Green Warblers at QUBS is among the most intriguing aspects of this species. Are individuals consistent is their habitat selection from year to year? Are there quantifiable differences between individuals using different habitats, migration timing, morphological characteristics, genetics, foraging behaviour and nesting characteristics? Further investigation of changes in migration phenology in this and other migrants is also warranted. Advancing arrival dates of spring migrants on the breeding grounds has been documented in many European species (i.e. Mason 1995; Jenkins and Watson 2000; Forchhammer et al 2002; Gilyazov and Sparks 2002; Cotton 2003; Huppop and Huppop 2003) and has been attributed to long-term climate warming. Long-term phenological records at QUBS would be advantageous in future climate-related research. The large difference in territory size among individual male black-throated green warblers at QUBS also deserves more attention. What factors influence territory size? Does fitness or annual return rates differ with territory size?

Literature & further reading:

Baicich, P.J. and Harrison, C.J.O. 1997. A guide to the nests, eggs and nestlings of North American Birds. Princeton University Press.